To support large numbers of (roaming) subscribers—e.g. mobile subscribers or nomadic, i.e. temporarily fixed subscribers—a number of mobile communication networks such as public land mobile networks and/or stationary (fixed line) networks have been established and are operative. Mobile communication networks such as public land mobile networks (PLMNs) may communicatively connect to other mobile communication networks and/or to fixed, land-wired public switched telephone networks (PSTNs) and/or to data networks such as the internet (or IP-networks, internet protocol networks). User equipments (e.g. a mobile or nomadic terminal) are typically assigned to a home mobile communication network, such as a home public land mobile network. In a roaming scenario, the user equipment is typically not located in reach of the home mobile communication network (or home public land mobile network) but is connected to a visited mobile communication network (or visited public land mobile network). Most modern mobile communication networks (such as public land mobile networks or also data networks) are packet-switched networks, in which data are routed in autonomous units called data packets. Packet-switched networks are characterized by high speed, low latency, and high network resource utilization efficiency. Two examples of packet-switched technologies are the General Packet Radio Service (GPRS) of the Global System for Mobile communications (GSM), and the Evolved Packet System (EPS), also commercially known as the Long Term Evolution (LTE), of the Universal Mobile Telecommunications System (UMTS). The Internet Protocol (IP) is a widely-deployed protocol for data communications in packet-switched networks. The IP Multimedia System (IMS) is an industry standard framework for delivering voice and multimedia communications over IP networks. Communications between nodes within an IMS network utilize the Session Initiation Protocol (SIP). SIP is a signaling protocol for Internet conferencing, telephony, presence, events notification, instant messaging, and the like. SIP signaling uses a long-term stable identifier, the SIP Universal Resource Indicator (URI).
Nowadays, numerous operators of mobile communication networks have already launched VoLTE functionality (i.e. Voice over LTE (Long Term Evolution) networks). This also means that the importance of VoLTE roaming is continuously increasing.
For providing VoLTE roaming, different solutions exist, including S8HR (S8 Home Routing) and LBO (Local Breakout) scenarios. Unlike LBO, S8HR does not require IMS network nodes in the visited mobile communication network (i.e. the visited LTE network) and therefore, IMS interoperability testing is not required between the home mobile communication network and the visited mobile communication network, which significantly simplifies implementation. This means that the main issue using a local breakout IMS roaming architecture is the impact on the visited network IMS nodes supporting roaming user equipments (i.e. typically P-CSCF (Proxy-Call Session Control Function), PCRF (Policy and Charging Rules Function), IBCF (Interconnection Border Control Function)), which causes high effort in integration, setup, and testing. However, S8HR is also a source of some serious technical issues, including service continuity problems (especially voice call continuity), as well as drawbacks regarding functionalities such as lawful interception, handover delays, non-authorized user emergency calls for visiting end users of the visited mobile communication network, and deploying existing voice charging models.